In his latest post Derick Rethans (knower of all things date and time) talks about the DateTimeImmutable functionality. It has been added into the PHP 5.5 releases and provides the same DateTime functionality but removes the ability for modification (mutability).

The first time that my improved DateTime support made its way into PHP was officially in PHP 5.1, although the more advanced features such as the DateTime class only made it appearance in PHP 5.2. Since its introduction the DateTime class implementation suffered from one design mistake - arguably not something that even an RFC would have highlighted. [...] This mutability property that all modifying methods of the DateTime class have is highly annoying, and something that I would now rather remove. But of course we cannot as that would break backwards compatibility. So in PHP 5.5, after a few stumbles, I finally managed to rectify this.

He includes some code examples showing the current DateTime object's mutability (via the "modify" function) and the new immutable handling. This new handling doesn't update the current object but instead returns the modified object, leaving the initial one intact. You can find out more about this new object in the PHP manual.